The present invention relates to a steering system for agricultural tractors and more particularly to a steering control system which automatically positions the tractor drive wheels for crab steering in response to implement side draft.
For many years, most agricultural tractors were controlled through the use of a steering wheel that controlled the turning movement for the front pair of wheels. Because of the substantially increased size of agricultural tractors in recent years, other steering systems have been developed that are more versatile than conventional front wheel steering. For example, four wheel steering systems have been developed wherein the respective wheels can be individually controlled to have various modes of steering control for the tractor.
One such steering system utilized by the assignee of the present invention provides for several modes of steering operation for the tractor. The first mode is referred to as front wheel steering where the rear wheels are locked in a set position with respect to the frame of the tractor and the front wheels are steerable. The second mode of steering is referred to as four wheel steering where the front and rear wheels are turned an equal amount but in opposite directions providing coordinated steering for sharp turns. A third mode of steering is commonly referred to as crab steering where the front and rear wheels are angularly disposed with respect to the longitudinal axis of the tractor at equal angles in the same direction so that the longitudinal axis of the tractor is moved in a generally sidewise direction. An additional mode of steering allows for independent control of the rear wheels when the tractor is being operated in a crowded area or when the tractor is being moved towards an implement for hook up.
U.S. Pat. Nos. 3,903,983 and 4,175,638, assigned to the assignee of the present invention, are illustrative of coordinated steering systems wherein the front and rear wheels can be independently controlled and the rear wheels positioned as a function of the angular position of the front wheels. While these systems have been found to be commercially acceptable for controlling the turning movement of an agricultural tractor having four driven wheels, there is a need for even more versatile systems which can be incorporated into the basic structure of the tractor without extensive modification thereof.
Crab steering is a mode of steering operation which could be improved in presently known systems. As described, crab steering permits the tractor and attached implement to move laterally or sideways for going around a wet spot in a field, closing up an overlap in the field, working closer to fence rows and the like. When working on hillsides, crab steering permits the wheels to be turned up slope to offset any implement side draft or jackknifing which reduces the possibility of the tractor sliding down the hill. A problem with known systems is that the operator may not be able to react quickly enough to counteract the undesirable tractor sliding because he must perform certain manual procedures before the crab steering mode can be actuated. Thus, there has been a need for a more versatile steering control system which will automatically position the tractor drive wheels for crab steering in response to implement side draft.